Trim the cucumber ends and slice into rounds about ⅛ inch (3 mm) thick. Toss with the measured salt in a colander set over a bowl. Refrigerate for 20 minutes, turning the slices once halfway through.
While the cucumbers drain, cover the red onion slices in cold water for 5 minutes, then drain and pat dry.
Combine the vinegar, olive oil, sugar, lemon zest, and pepper in a small jar. Close and shake until the sugar dissolves.
Discard the drained cucumber liquid. Blot the cucumber rounds gently on a clean towel, pressing lightly without tearing.
Toss the cucumbers, dried onion, and dill in a bowl with the shaken dressing until lightly coated.
Cover and refrigerate for 10 minutes. Lift and turn the slices once, then taste before adjusting. Serve with a slotted spoon if liquid has collected underneath.